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Test Structures (test + structure)

Distribution by Scientific Domains

Engineering	46%
Chemistry	32%
Polymers and Materials Science	10%
Physics and Astronomy	10%

Selected Abstracts

Investigation of phase contrast hard X-ray microscopy using planar sets of refractive crossed linear parabolic lenses made from SU-8 polymer

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 8 2007
E. Reznikova
Abstract Planar X-ray refractive lenses with parabolic surface profile, and in crossed geometry to provide 2D focusing, are fabricated from SU-8 polymer using the LIGA process and deep X-ray lithography technology. A transmission X-ray microscope (TXM) using a condenser and an objective lens based on this type of X-ray optics was set up at the ESRF beamline BM 5, for photon energies of 17.1 and 18 keV. Test structures made of gold and SU-8, with different thicknesses, were imaged with this TXM using in-line phase-contrast, with X-ray magnification factors of 13,20, spatial resolution between 0.2 and 0.3 µm and exposure times around 1 s. The advantages of a TXM based on refractive SU-8 planar crossed condenser and objective and the optimisation of the optical scheme and of the condenser focusing profile are discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Uncertainty and Sensitivity Analysis of Damage Identification Results Obtained Using Finite Element Model Updating

COMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 5 2009
Babak Moaveni
The shake table tests were designed so as to damage the building progressively through several historical seismic motions reproduced on the shake table. A sensitivity-based finite element (FE) model updating method was used to identify damage in the building. The estimation uncertainty in the damage identification results was observed to be significant, which motivated the authors to perform, through numerical simulation, an uncertainty analysis on a set of damage identification results. This study investigates systematically the performance of FE model updating for damage identification. The damaged structure is simulated numerically through a change in stiffness in selected regions of a FE model of the shear wall test structure. The uncertainty of the identified damage (location and extent) due to variability of five input factors is quantified through analysis-of-variance (ANOVA) and meta-modeling. These five input factors are: (1,3) level of uncertainty in the (identified) modal parameters of each of the first three longitudinal modes, (4) spatial density of measurements (number of sensors), and (5) mesh size in the FE model used in the FE model updating procedure (a type of modeling error). A full factorial design of experiments is considered for these five input factors. In addition to ANOVA and meta-modeling, this study investigates the one-at-a-time sensitivity analysis of the identified damage to the level of uncertainty in the identified modal parameters of the first three longitudinal modes. The results of this investigation demonstrate that the level of confidence in the damage identification results obtained through FE model updating, is a function of not only the level of uncertainty in the identified modal parameters, but also choices made in the design of experiments (e.g., spatial density of measurements) and modeling errors (e.g., mesh size). Therefore, the experiments can be designed so that the more influential input factors (to the total uncertainty/variability of the damage identification results) are set at optimum levels so as to yield more accurate damage identification results. [source]

Kinematic transformations for planar multi-directional pseudodynamic testing

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 9 2009
Oya Mercan
Abstract The pseudodynamic (PSD) test method imposes command displacements to a test structure for a given time step. The measured restoring forces and displaced position achieved in the test structure are then used to integrate the equations of motion to determine the command displacements for the next time step. Multi-directional displacements of the test structure can introduce error in the measured restoring forces and displaced position. The subsequently determined command displacements will not be correct unless the effects of the multi-directional displacements are considered. This paper presents two approaches for correcting kinematic errors in planar multi-directional PSD testing, where the test structure is loaded through a rigid loading block. The first approach, referred to as the incremental kinematic transformation method, employs linear displacement transformations within each time step. The second method, referred to as the total kinematic transformation method, is based on accurate nonlinear displacement transformations. Using three displacement sensors and the trigonometric law of cosines, this second method enables the simultaneous nonlinear equations that express the motion of the loading block to be solved without using iteration. The formulation and example applications for each method are given. Results from numerical simulations and laboratory experiments show that the total transformation method maintains accuracy, while the incremental transformation method may accumulate error if the incremental rotation of the loading block is not small over the time step. A procedure for estimating the incremental error in the incremental kinematic transformation method is presented as a means to predict and possibly control the error. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Seismic performance of a 3D full-scale high-ductility steel,concrete composite moment-resisting structure,Part I: Design and testing procedure

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 14 2008
A. Braconi
Abstract A multi-level pseudo-dynamic (PSD) seismic test programme was performed on a full-scale three-bay two-storey steel,concrete composite moment-resisting frame built with partially encased composite columns and partial-strength connections. The system was designed to provide strength and ductility for earthquake resistance with energy dissipation located in ductile components of beam-to-column joints including flexural yielding of beam end-plates and shear yielding of the column web panel zone. In addition, the response of the frame depending on the column base yielding was analysed. Firstly, the design of the test structure is presented in the paper, with particular emphasis on the ductile detailing of beam-to-column joints. Details of the construction of the test structure and the test set-up are also given. The paper then provides a description of the non-linear static and dynamic analytical studies that were carried out to preliminary assess the seismic performance of the test structure and establish a comprehensive multi-level PSD seismic test programme. The resulting test protocol included the application of a spectrum-compatible earthquake ground motion scaled to four different peak ground acceleration levels to reproduce an elastic response as well as serviceability, ultimate, and collapse limit state conditions, respectively. Severe damage to the building was finally induced by a cyclic test with stepwise increasing displacement amplitudes. Copyright © 2008 John Wiley & Sons, Ltd. [source]

A buffered impact damper for multi-degree-of-freedom structural control

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 13 2008
Kuinian Li
Abstract The results of an experimental investigation into the use of a buffered impact damper for controlling the dynamic response of an MDOF (multi-degree-of-freedom) structure are presented in this paper. Free and forced vibration tests of a three-DOF test structure equipped with a buffered impact damper are performed to evaluate the resulting damping effect and impact characteristics. The effect of damper parameters, such as clearance, and excitation type on the performance of the impact damper is also investigated. The performance of the buffered impact damper is compared with that of an equivalent conventional rigid impact damper. It is found that the buffered impact damper not only significantly reduces the peak contact force and the associated accelerations and noise generated by collisions but can also substantially enhance the damping effect over a wide range of frequencies, encompassing the natural frequencies of the test structure. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Stability analysis for real-time pseudodynamic and hybrid pseudodynamic testing with multiple sources of delay

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 10 2008
Oya Mercan
Abstract Real-time pseudodynamic (PSD) and hybrid PSD test methods are experimental techniques to obtain the response of structures, where restoring force feedback is used by an integration algorithm to generate command displacements. Time delays in the restoring force feedback from the physical test structure and/or the analytical substructure cause inaccuracies and can potentially destabilize the system. In this paper a method for investigating the stability of structural systems involved in real-time PSD and hybrid PSD tests with multiple sources of delay is presented. The method involves the use of the pseudodelay technique to perform an exact mapping of fixed delay terms to determine the stability boundary. The approach described here is intended to be a practical one that enables the requirements for a real-time testing system to be established in terms of system parameters when multiple sources of delay exist. Several real-time testing scenarios with delay that include single degree of freedom (SDOF) and multi-degree of freedom (MDOF) real-time PSD/hybrid PSD tests are analyzed to illustrate the method. From the stability analysis of the real-time hybrid testing of an SDOF test structure, delay-independent stability with respect to either experimental or analytical substructure delay is shown to exist. The conditions that the structural properties must satisfy in order for delay-independent stability to exist are derived. Real-time hybrid PSD testing of an MDOF structure equipped with a passive damper is also investigated, where observations from six different cases related to the stability plane behavior are summarized. Throughout this study, root locus plots are used to provide insight and explanation of the behavior of the stability boundaries. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Test on full-scale three-storey steel moment frame and assessment of ability of numerical simulation to trace cyclic inelastic behaviour

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 1 2006
Masayoshi Nakashima
Abstract A test on a full-scale model of a three-storey steel moment frame was conducted, with the objectives of acquiring real information about the damage and serious strength deterioration of a steel moment frame under cyclic loading, studying the interaction between the structural frame and non-structural elements, and examining the capacity of numerical analyses commonly used in seismic design to trace the real cyclic behaviour. The outline of the test structure and test program is presented, results on the overall behaviour are given, and correlation between the experimental results and the results of pre-test and post-test numerical analyses is discussed. Pushover analyses conducted prior to the test predicted the elastic stiffness and yield strength very reasonably. With proper adjustment of strain hardening after yielding and composite action, numerical analyses were able to accurately duplicate the cyclic behaviour of the test structure up to a drift angle of 1/25. The analyses could not trace the cyclic behaviour involving larger drifts in which serious strength deterioration occurred due to fracture of beams and anchor bolts and progress of column local buckling. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Adaptive bang,bang control for the vibration control of structures under earthquakes

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 13 2003
C. W. Lim
Abstract An adaptive method based on the modified bang,bang control algorithm is proposed for the vibration control of structures subjected to unexpected severe seismic loads greater than the design loads. A hydraulic-type active mass damper was made and experiments were carried out in the laboratory using a one-story test structure and a five-story test structure with the active mass damper. Through numerical simulations and experiments it was confirmed that the proposed method works well to suppress the vibration of structures subjected to unexpected severe seismic loads greater than the design loads without causing any unstable situations. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Study on effects of damping in laminated rubber bearings on seismic responses for a , scale isolated test structure

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 10 2002
Bong Yoo
Abstract The effects of damping in various laminated rubber bearings (LRB) on the seismic response of a ,-scale isolated test structure are investigated by shaking table tests and seismic response analyses. A series of shaking table tests of the structure were performed for a fixed base design and for a base isolation design. Two different types of LRB were used: natural rubber bearings (NRB) and lead rubber bearings (LLRB). Three different designs for the LLRB were tested; each design had a different diameter of lead plug, and thus, different damping values. Artificial time histories of peak ground acceleration 0.4g were used in both the tests and the analyses. In both shaking table tests and analyses, as expected, the acceleration responses of the seismically isolated test structure were considerably reduced. However, the shear displacement at the isolators was increased. To reduce the shear displacement in the isolators, the diameter of the lead plug in the LLRB had to be enlarged to increase isolator damping by more than 24%. This caused the isolator stiffness to increase, and resulted in amplifying the floor acceleration response spectra of the isolated test structure in the higher frequency ranges with a monotonic reduction of isolator shear displacement. Copyright © 2002 John Wiley & Sons, Ltd. [source]

A displacement-based seismic design procedure for RC buildings and comparison with EC8

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 10 2001
T. B. Panagiotakos
Abstract A procedure for displacement-based seismic design (DBD) of reinforced concrete buildings is described and applied to a 4-storey test structure. The essential elements of the design procedure are: (a) proportioning of members for gravity loads; (b) estimation of peak inelastic member deformation demands in the so-designed structure due to the design (,life-safety') earthquake; (c) revision of reinforcement and final detailing of members to meet these inelastic deformation demands; (d) capacity design of members and joints in shear. Additional but non-essential steps between (a) and (b) are: (i) proportioning of members for the ULS against lateral loads, such as wind or a serviceability (,immediate occupancy') earthquake; and (ii) capacity design of columns in flexure at joints. Inelastic deformation demands in step (b) are estimated from an elastic analysis using secant-to-yield member stiffnesses. Empirical expressions for the deformation capacity of RC elements are used for the final proportioning of elements to meet the inelastic deformation demands. The procedure is applied to one side of a 4-storey test structure that includes a coupled wall and a two-bay frame. The other side is designed and detailed according to Eurocode 8. Major differences result in the reinforcement of the two sides, with significant savings on the DBD-side. Pre-test calculations show no major difference in the seismic performance of the two sides of the test structure. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Active tendon control of cable-stayed bridges: a large-scale demonstration

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 7 2001
Frédéric Bossens
This paper presents a strategy for active damping of cable structures, using active tendons. The first part of the paper summarizes the theoretical background: the control law is briefly presented together with the main results of an approximate linear theory which allows the prediction of closed-loop poles with a root locus technique. The second part of the paper reports on experimental results obtained with two test structures: the first one is a small size mock-up representative of a cable-stayed bridge during the construction phase. The control of the parametric vibration of passive cables due to deck vibration is demonstrated. The second one is a 30 m long mock-up built on the reaction wall of the ELSA test facility at the JRC Ispra (Italy); this test structure is used to demonstrate the practical implementation of the control strategy with hydraulic actuators. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Experimental evidence of parasitic shunting in silicon nitride rear surface passivated solar cells

PROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 4 2002
Stefan Dauwe
Many solar cells incorporating SiNx films as a rear surface passivation scheme have not reached the same high level of cell performance as solar cells incorporating high-temperature-grown silicon dioxide films as a rear surface passivation. In this paper, it is shown by direct comparison of solar cells incorporating the two rear surface passivation schemes, that the performance loss is mainly due to a lower short-circuit current while the open-circuit voltage is equally high. With a solar cell test structure that features a separation of the rear metal contacts from the passivating SiNx films, the loss in short-circuit current can be reduced drastically. Besides a lower short- circuit current, dark I,V curves of SiNx rear surface passivated solar cells exhibit distinct shoulders. The results are explained by parasitic shunting of the induced floating junction (FJ) underneath the SiNx films with the rear metal contacts. The floating junction is caused by the high density of fixed positive charges in the SiNx films. Other two-dimensional effects arising from the injection level dependent SRV of the Si/SiNx interfaces are discussed as well, but, are found to be of minor importance. Pinholes in the SiNx films and optical effects due to a different internal rear surface reflectance can be excluded as a major cause for the performance loss of the SiNx rear surface passivated cells. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Active tendon control of cable-stayed bridges: a large-scale demonstration

Determination of Mechanical Properties of Copper at the Micron Scale,

ADVANCED ENGINEERING MATERIALS, Issue 11 2006
D. Kiener
Using a focused ion beam workstation, micron-sized bending and compression samples were fabricated from a pure copper single crystal. The bending and compression experiments exhibited a strong size effect on the flow stress of copper, reaching values in the order of 1,GPa for the smallest test structures. Conventional strain gradient plasticity approaches are not capable of explaining this behaviour. The surface damage introduced by Ga+ ion implantation during focused ion beam preparation was investigated using Auger electron spectroscopy and its consequence on the mechanical response of the miniaturized test samples is addressed. [source]

Characterization of strength properties of thin polycrystalline silicon films for MEMS applications

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 1 2007
R. Boroch
ABSTRACT The aim of this work is to characterize the strength properties of polycrystalline silicon (polysilicon) with the use of tensile and bending test specimens. The strength of thin polysilicon films with different geometry, size and stress concentrations has been measured and correlated with the effective size of the specimen and its stress distribution. The test results are evaluated using a probabilistic strength approach based on the weakest link theory with the use of STAU software. The use of statistic methods of strength prediction of polysilicon test structures with a complex geometry and loading based on test values for standard material tests specimen has been evaluated. [source]

Advances in powder diffraction pattern indexing: N-TREOR09

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2009
Angela Altomare
Powder pattern indexing can still be a challenge, despite the great recent advances in theoretical approaches, computer speed and experimental devices. More plausible unit cells, belonging to different crystal systems, are frequently found by the indexing programs, and recognition of the correct one may not be trivial. The task is, however, of extreme importance: in case of failure a lot of effort and computing time may be wasted. The classical figures of merit for estimating the unit-cell reliability {i.e.M20 [de Wolff (1968). J. Appl. Cryst.1, 108,113] and FN [Smith & Snyder (1979). J. Appl. Cryst.12, 60,65]} sometimes fail. For this reason, a new figure of merit has been introduced in N-TREOR09, the updated version of the indexing package N-TREOR [Altomare, Giacovazzo, Guagliardi, Moliterni, Rizzi & Werner (2000). J. Appl. Cryst. 33, 1180,1186], combining the information supplied by M20 with additional parameters such as the number of unindexed lines, the degree of overlap in the pattern (the so-called number of statistically independent observations), the symmetry deriving from the automatic evaluation of the extinction group, and the agreement between the calculated and observed profiles. The use of the new parameters requires a dramatic modification of the procedures used worldwide: in the approach presented here, extinction symbol and unit-cell determination are simultaneously estimated. N-TREOR09 benefits also from an improved indexing procedure in the triclinic system and has been integrated into EXPO2009, the updated version of EXPO2004 [Altomare, Caliandro, Camalli, Cuocci, Giacovazzo, Moliterni & Rizzi (2004). J. Appl. Cryst. 37, 1025,1028]. The application of the new procedure to a large set of test structures is described. [source]

Advances in space-group determination from powder diffraction data

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2007
Angela Altomare
The space-group determination process by powder diffraction data is not straightforward. The low accuracy of the reflection intensities may invalidate the calculation of the probability associated to each extinction group that is compatible with the crystal system determined in the indexation step. Here the combination of the z statistics with two new algorithms is reported: the first checks the quality of each 2, interval in order to omit doubtful z estimates from the calculations; the second creates a list of reflections with peaks that weakly overlap with any other peak, in order to check if any of them violates the extinction rules of the extinction symbol. The new approach has been applied to a large set of test structures and proved to be much more efficient than the procedure based only on the z statistics. [source]

The revenge of the Patterson methods.

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2007

The Patterson techniques, recently developed by the same authors for the ab initio crystal structure solution of proteins, have been applied to single and multiple anomalous diffraction (SAD and MAD) data to find the substructure of the anomalous scatterers. An automatic procedure has been applied to a large set of test structures, some of which were originally solved with remarkable difficulty. In all cases, the procedure automatically leads to interpretable electron density maps. Patterson techniques have been compared with direct methods; the former seem to be more efficient than the latter, so confirming the results obtained for ab initio phasing, and disproving the common belief that they could only be applied to determine large equal-atom substructures with difficulty. [source]

About the efficiency of the early FOMs in ab initio protein phasing

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2004
Maria C. Burla
All ab initio techniques for solving protein crystal structures use multisolution approaches. Several figures of merit that are found in the literature are efficient in the last steps of the phasing process, when some trials converge to the correct solution with a relatively small average phase error. Early figures of merit are much more critical; they should be able to recognize useful trials when the phase error is still large, and their efficiency determines the efficiency of the program. In the present work, a wide variety of known figures of merit at atomic and quasi-atomic (,1.4,Å) resolution have been tested; new figures have also been devised and tested. Their application to a large set of test structures allows the study of their properties at different data resolutions and the selection of the most efficient figures within the SIR2003-N framework. [source]

Completion of crystal structures from powder data: the use of the coordination polyhedra

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2000
Angela Altomare
Direct methods applied to powder diffraction data often provide well located heavy atoms and unreliable light-atom positions. The completion of the crystal structure is then not always straightforward and may require a considerable amount of user intervention. The heavy-atom connectivity provided by the trial solution may be used to guess the nature of the coordination polyhedra. A Monte Carlo procedure is described which, in the absence of a well defined structural model, is able to locate the light atoms correctly under the restraints of the experimental heavy-atom connectivity model. The correctness of the final model is assessed by criteria based on the agreement between the whole experimental diffraction pattern and the calculated one. The procedure requires little CPU computing time and has been implemented as a routine of EXPO [Altomare et al. (1999). J. Appl. Cryst.32, 339,340]. The method has proved to be sufficiently robust against the distortion of the coordination polyhedra and has been successfully applied to some test structures. [source]

Influence of top layer geometries on the electronic properties of pentacene and diindenoperylene thin films

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2008
M. Scharnberg
Abstract Top layers have a pronounced influence on the electronic properties of molecular organic thin films. Here, we report about the changes induced by metallic and polymeric top layers and contacts. As test structures, model systems of diindenoperylene and pentacene crystalline molecular organic thin films are used. A very sensitive radiotracer technique is introduced to study the details of metal penetration during top contact formation. The influence of temperature, evaporation time, adhesion promoter and grain size of the organic film were examined. The electric currents passing through metal top contacts were found to vary by more than a factor of three, depending on the preparation conditions of the metal contact. Furthermore, the series resistance of chemically identical contacts that only differed in the morphology of the interface were found to show pronounced asymmetric conductivity behaviour. We also show that with the help of electret top layers, based on the Teflon-AF fluropolymer, the threshold voltage of an organic field effect transistor can be tuned by several volts. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Magnetotransport in AlGaN/GaN and AlGaN/AlN/GaN heterostructures

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2007
G. A. Umana-Membreno
Abstract Two-dimensional electron gas transport in AlN/GaN and AlGaN/GaN heterostructures has been investigated employing geometrical magnetoresistance measurements and quantitative mobility-spectrum analysis. The channel magnetoresistance of ungated four-terminal test structures, with an effective width to length ratio of 10, was measured using pulsed drain-to-source voltages equivalent to longitudinal electric fields up to 750 V/cm at magnetic field intensities up to 12 T and sample temperatures from 10 to 300 K. Two distinct electron populations, with significantly different mobilities, are shown to be present in the channel of both AlGaN/AlN/GaN and AlGaN/GaN heterostructures. It is also shown that application of longitudinal electric fields up to 750 V/cm cause a reduction in the mobility of these carrier populations and change the shape of mobility spectrum of the dominant electron population. The origin of these carrier populations is discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Influence of Hydrogen Plasma on the Defect Passivation of Polycrystalline Si Thin Film Solar Cells

PLASMA PROCESSES AND POLYMERS, Issue S1 2009
Benjamin Gorka
Abstract Hydrogen passivation (HP) of polycrystalline silicon (poly-Si) thin film solar cells was performed in a parallel plate radio-frequency (rf) plasma setup. The influence of hydrogen pressure p and electrode gap d on breakdown voltage Vbrk is presented showing that the minimum in Vbrk shifts with higher pressures towards higher p,·,d values. Cell test structures provided by CSG Solar AG were used to examine the influence of p and d on the open circuit voltage VOC. The highest VOC's were achieved for p,·,d values that correspond to a minimum in Vbrk. HP strongly improved the VOC. After the hydrogen plasma treatment the VOC improved significantly by a factor of 2 and amounted to 450 mV. Optimized parameters were then applied to different poly-Si solar cells prepared by electron beam evaporation. [source]

Molecular replacement: the probabilistic approach of the program REMO09 and its applications

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2009
Rocco Caliandro
The method of joint probability distribution functions has been applied to molecular replacement techniques. The rotational search is performed by rotating the reciprocal lattice of the protein with respect to the calculated transform of the model structure; the translation search is performed by fast Fourier transform. Several cases of prior information are studied, both for the rotation and for the translation step: e.g. the conditional probability density for the rotation or the translation of a monomer is found both for ab initio and when the rotation and/or the translation values of other monomers are given. The new approach has been implemented in the program REMO09, which is part of the package for global phasing IL MILIONE [Burla, Caliandro, Camalli, Cascarano, De Caro, Giacovazzo, Polidori, Siliqi & Spagna (2007). J. Appl. Cryst.40, 609,613]. A large set of test structures has been used for checking the efficiency of the new algorithms, which proved to be significantly robust in finding the correct solutions and in discriminating them from noise. An important design concept is the high degree of automatism: REMO09 is often capable of providing a reliable model of the target structure without any user intervention. [source]

The (Fo,Fc) Fourier synthesis: a probabilistic study

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2008
Rocco Caliandro
(Fo,Fc) and (2Fo,Fc) Fourier syntheses are considered the most powerful tools for recovering the remainder of a structure and for correcting crystal structure models. A probabilistic approach has been applied to derive the formula for the variance for the expected value of the coefficient (Fo,Fc). This has allowed a better understanding of the features of the difference Fourier synthesis; in particular, a subset of well phased reflections has been separated from the subset of reflections best phased by the standard Fo Fourier synthesis. An iterative procedure, based on the electron-density modification of the difference Fourier map, has been devised which aims to improve phase and modulus estimates of the reflections with higher variance value, by using as lever arm the set of reflections with lower variance value. The new procedure (DEDM) has been implemented and verified on a wide set of test structures, the partial models of which were obtained by molecular replacement or by automatic model-building routines applied to experimental electron-density maps. Phase and modulus estimates of the difference Fourier syntheses improve in all the test cases; as a consequence, the quality of the difference Fourier maps also improves in the region where the target structure deviates from the partial model. A new procedure is suggested, combining DEDM with standard electron-density modification techniques, which leads to significant reduction of the phase errors. The procedure may be considered a starting point for further developments. [source]

The pair-functional method for direct solution of molecular structures.

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2001

The new pair-functional direct method has been implemented and tested. Like the Patterson function, the pairing force has valuable imaging properties at high resolution. Two simple iterative algorithms were designed to refine on the total pair potential and the normalized intensity correlation coefficient of an atomic model. The first algorithm is a peak-picking method which selects the best-paired high peaks from a density map and then uses the strong reflections to generate a new Fourier filtered map. The second algorithm, the pair-and-square method, uses a tangent formula step instead of the Fourier and is a little more efficient. Computational experiments on a point-atom grid model, with perfect data, reached exact ab initio solutions for up to 600 atoms. Point-atom models were also solved by searching for reduced structures that contained as few as one quarter of the atoms. Seeded searches, guided by a small known fragment, solved up to 30000 atoms on the grid. Realistic tests on actual molecules showed that Sheldrick's [Acta Cryst. (1990), A46, 467,473] test structures of 50,200 atoms can be solved under a variety of conditions. [source]

MAD phasing: choosing the most informative wavelength combination

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 9 2004
Maria Cristina Burla
Two algorithms are described for limiting data resolution and for predicting the most informative wavelength combinations in MAD techniques. Both have been successfully tested using experimental data from a large set of test structures. [source]